Field of the Invention
The present disclosure relates to an organic light emitting diode display and a method of driving the same.
Discussion of the Related Art
An active matrix organic light emitting diode display includes organic light emitting diodes (OLEDs) capable of emitting light by themselves and has many advantages, such as a fast response time, a high emission efficiency, a high luminance, a wide viewing angle, and the like.
An OLED serving as a self-emitting element includes an anode electrode, a cathode electrode, and an organic compound layer between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a power voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML and form excitons. As a result, the emission layer EML generates visible light.
An organic light emitting diode display arranges pixels each including an OLED in a matrix form and adjusts a luminance of the pixels based on a grayscale of video data. Each pixel includes a driving thin film transistor (TFT) controlling a driving current flowing in the OLED based on a voltage between a gate electrode and a source electrode of the driving TFT, and at least one switching TFT programming the gate-to-source voltage of the driving TFT. Each pixel adjusts the display grayscale (luminance) by an amount of emitted light of the OLED which is proportional to the driving current.
In such an organic light emitting diode display, a duty control technique for adjusting an emission duty in one frame has been proposed in order to improve video response characteristics and low grayscale display quality.
According to a related art, a duty control technique 1, as shown in FIG. 1, divides one frame (Fn+1 or Fn+2) into an emission period Ta and a black display period Tb and writes black data according to a line sequential manner at a predetermined timing to control the black display period Tb. The black data has a data level capable of turning off a driving TFT. When the black data is applied, a driving current applied to the OLED is cut off, so that the OLED is not emitted. As a timing for writing the black data in one frame is advanced, the emission period Ta is decreased and the black display period Tb is increased. According to this duty control technique 1, an output channel potential of the data driving circuit must continuously swing from the video data level to the black data level or vice versa for black data writing. Thus, there is a problem that power consumed and heat generated in the data driving circuit are increased.
A duty control technique 2 according to a related art, as shown in FIG. 2, further includes a separate emission control TFT ET in a pixel and, and divides one frame (Fn+1 or Fn+2) into an emission period Ta and a black display period Tb as shown in FIG. 1. The duty control technique 2 turns off the emission control TFT ET according to a line sequential manner at a predetermined timing to realize the black display period Tb. The emission control TFT ET may be connected to an arbitrary position between an input terminal of a high potential driving voltage EVDD and an input terminal of a low potential driving voltage EVSS in the pixel. In FIG. 2, DT indicates a driving TFT, and SWC indicates a switching circuit connected to the driving TFT DT and the emission control TFT ET. When the emission control TFT ET is turned off, a driving current applied to the OLED is cut off, so that the OLED is not emitted. The duty control technique 2 has a problem that the pixel array configuration becomes complicated because the emission control TFT ET is added to each pixel. The duty control technique 2 has a problem that luminance distortion occurs due to a kick back effect by a parasitic capacitance when the emission control TFT ET is turned off.